Automating Manufacturing Systems - Process Control and ...

plc states - 12.6
Previously state diagrams were used to define the system, it is possible to use a
state table as shown in Figure 12.6. Here the light sequences are listed in order. Each state
is given a name to ease interpretation, but the corresponding output pattern is also given.
The system state is defined as the bit pattern of the 6 lights. Note that there are only 4 patterns,
but 6 binary bits could give as many as 64.
Step 1: Define the System States and put them (roughly) in sequence
State Description
Green East/West
Yellow East/West
Green North/South
Yellow North/South
System State
L1 L2 L3 L4 L5 L6
State Table
# L1 L2 L3 L4 L5 L6
1
2
3
4
1
1
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
1
0
0
0
A binary number
0 = light off
1 = light on
Here the four states
determine how the 6
outputs are switched
on/off.
Figure 12.6
System State Table for Traffic Lights
Transitions can be added to the state table to clarify the operation, as shown in Figure
12.7. Here the transition from Green E/W to Yellow E/W is S1. What this means is
that a cross walk button must be pushed to end the green light. This is not normal, normally
the lights would use a delay. The transition from Yellow E/W to Green N/S is
caused by a 4 second delay (this is normal.) The next transition is also abnormal, requiring
that the cross walk button be pushed to end the Green N/S state. The last state has a 4 second
delay before returning to the first state in the table. In this state table the sequence will
always be the same, but the times will vary for the green lights.